Synthesis of Rh(I) alkylated-PTA complexes as catalyst precursors in the aqueous-biphasic hydroformylation of 1-octene

Article abstract of DOI:10.1016/j.jorganchem.2018.05.019

A series of mono- and multivalent phosphorus-based ligands was synthesised by the lower-rim benzylation of the water-soluble ligand, 1,3,5-triaza-7-phosphaadamantane (PTA). These ligands were used to prepare mono-, bi- and trinuclear Rh(I) complexes whose catalytic activity, product selectivity and recyclability were evaluated in the aqueous biphasic hydroformylation of 1-octene.

Full text of DOI:10.1016/j.jorganchem.2018.05.019

Accepted Manuscript
Synthesis of Rh(I) alkylated-PTA complexes as catalyst precursors in the aqueous-
biphasic hydroformylation of 1-octene
Diteboho S. Ramarou, Banothile C.E. Makhubela, Gregory S. Smith
PII:
S0022-328X(18)30400-5
10.1016/j.jorganchem.2018.05.019
JOM 20453
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 14 March 2018
Revised Date: 17 May 2018
Accepted Date: 23 May 2018
Please cite this article as: D.S. Ramarou, B.C.E. Makhubela, G.S. Smith, Synthesis of Rh(I) alkylated-
PTA complexes as catalyst precursors in the aqueous-biphasic hydroformylation of 1-octene, Journal of
Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.05.019.
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ACCEPTED MANUSCRIPT
Synthesis of Rh(I) alkylated-PTA complexes as catalyst precursors in the
aqueous-biphasic hydroformylation of 1-octene
Diteboho S. Ramaroua, Banothile C. E. Makhubelab, Gregory S. Smith*a
a Department of Chemistry, University of Cape Town, Rondebosch, 7701, South Africa
b
Department of Chemistry, University of Johannesburg, PO Box 524, Auckland Park, 2006, South Africa
Keywords: hydroformylation, polynuclear, aqueous biphasic, rhodium, PTA.
Abstract
A series of mono- and multivalent phosphorus-based ligands were synthesised by the lower-
rim benzylation of the water-soluble ligand, 1,3,5-triaza-7-phosphaadamantane (PTA). These
ligands were used to prepare mono-, bi- and trinuclear Rh(I) complexes whose catalytic
activity, product selectivity and recyclability were evaluated in the aqueous biphasic
hydroformylation of 1-octene.
Introduction
Catalysis plays a very important role in the chemical industry and aids in the production of
over 90% of all industrial chemicals produced [1,2]. Using a catalyst lowers the reaction’s
energy requirements, increases product selectivity and often reduces the use of processing
and separating agents [3–5]. The use of transition metals as catalysts has been vastly
explored [6]. Homogeneous catalysis has significant advantages over heterogeneous
catalysis because it displays greater activity and selectivity [7]. The major disadvantage with
homogeneous catalysis is separating the catalyst from the products obtained [8]. A number
of strategies [9–12] have been employed to combat the separation challenge, including
immobilising the catalyst using biphasic media [13]. In recent years, there has been a
considerable interest in the synthesis of water-soluble catalysts for use in a number of
reactions such as hydrogenation [14,15], carbonylation [16], alkene metathesis [17,18] and
hydroformylation [19,20].
The hydroformylation of olefins (Scheme 1) has grown to be an important industrial process
since the obtained aldehydes can be used to manufacture plasticisers, surfactants and
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detergents, chemical intermediates, solvents and lubricants [21]. The hydroformylation
reaction is an extensively studied process and much attention has been given to the design
and synthesis of new metal complexes that exhibit improved catalytic activity, greater
product selectivity and can easily be reused.
Scheme 1: Illustration of the hydroformylation reaction.
New metal complexes as catalysts for hydroformylation reactions continue to be studied
due to the need for simple and effective catalysts to facilitate complex catalytic reactions
such as in the aqueous biphasic hydroformylation of olefins. Aqueous biphasic
hydroformylation (Figure 1) combines the advantages of homogeneous and heterogeneous
catalyses and uses water as the second immiscible phase solvent. Innovative work into
aqueous biphasic catalysis can be traced back to the Ruhrchemie/Rhône-Poulenc (RCH/RP)
industrial process and several other academic reactions that make use of a highly water-
soluble rhodium complexes as catalysts for the hydroformylation of propene [22]. The use
of water is in line with Green Chemistry Principles due to water being eco-friendly, user-
friendly, non-toxic, abundant (hence relatively cheap) and non-flammable. Moreover, water
is immiscible with most organic solvents, addressing the challenge of catalyst recovery [8].
Vigorous stirring
Figure 1: Illustration of aqueous biphasic hydroformylation.
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To obtain water-soluble metal complexes, hydrophilic ligands are usually introduced into
the coordination sphere of the metal centre [23]. Water-soluble phosphines are the most
widely used [24], because they can be modified using a variety of organic substituents,
which in-turn affects the electronic and steric characteristics of the catalyst [25]. 1,3,5-
Triaza-7-phopsphaadamantane (PTA) is a water-soluble phosphine and its metal complexes
have been studied and found to be useful in catalysing many chemical reactions, including
the hydrogenation of olefinic and oxo acids [26], isomerisation and condensation of allyl
alcohol [27] and the reduction of aldehydes [28,29].
A more recent approach to improving the catalyst’s activity has been to increase the
number of metal centres. This motivation comes from nature’s highly selective
metalloenzymes. Having more than one metal centre introduces cooperative interactions
that may exist between proximate metal centres, enhancing catalytic activity compared to
the mononuclear analogues [30]. Park and Hong suggested that the ideal distance for
cooperativity between two metal centres is within 3.5 - 6 Å, with the exception of those
which exhibit a direct metal-metal bond. Thus, if the metals are an appropriate distance
from each other, higher reactivity and selectivity can be achieved as compared to when
using the monometallic complexes [31].
Coordinating highly water-soluble ligands, such as PTA, to the rhodium metal centre and
increasing the number of metal centres (rhodium atoms) per complex may further improve
catalytic rates, product selectivity and recovery of the catalyst in hydroformylation
reactions.
In this work, the syntheses and characterisation of a series of PTA ligands is reported. The
mono-, bi- and trimeric phosphine-based ligands were synthesised by the alkylation of PTA.
These ligands were then reacted with the appropriate equivalent of the rhodium precursor
[RhCl(COD)]2 (COD = 1,5-cyclooctadiene) to afford the corresponding mono-, bi- and
trinuclear Rh(I)-PTA complexes. The resulting complexes were then reacted with carbon
monoxide gas to yield the dicarbonyl analogues of the Rh(I)-PTA complexes. All complexes
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were then evaluated as catalyst precursors in aqueous biphasic hydroformylation of
1-octene.
Results and discussion
Synthesis and characterisation of ligands 1 – 3 and 1’ – 3’
The benzylated PTA derivatives were prepared as previously described in literature [32–34].
The benzylic halides (benzyl chloride, 1,4-bis(chloromethyl)benzene, and 1,3,5-
tris(chloromethyl)benzene) were reacted with appropriate equivalents of PTA to afford
ligands 1, 2, and 3 respectively (Scheme 2). Ligands 1 and 2 were isolated as white solids and
ligand 3 as a beige solid in moderate to good yields (38 – 97 %) and were all water-soluble.
The signals in the 1H NMR correlate with those reported in literature [32–34].
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Scheme 2: Syntheses of the monomeric, dimeric and trimeric ligands.
Complexation reactions were attempted by reacting ligands 1, 2 and 3 with [RhCl(COD)]2
and the resulting products were found to be unstable. A counterion exchange was then
performed, replacing the chlorido anions with the much larger hexaflurophosphate anions.
Ligands 1’, 2’ and 3’ were prepared by reacting each of ligands 1 – 3 with the appropriate
equivalent of NH4PF6 in ethanol (Scheme 2) and were obtained as white solids in moderate
1
to good yields (51 – 94 %). The proton signals observed in the H NMR spectra of ligands
1’ – 3’ show a similar splitting pattern as in the 1H NMR spectra of ligands 1 – 3. In addition,
1
a slight upfield shift of the PTA proton signals when compared to the H NMR spectra of
ligands 1 – 3 was observed, confirming successful anion exchange. Furthermore, the 31P{1H}
NMR spectra also confirms successful anion exchange, as two phosphorus entities are
observed for each of ligands 1’ – 3’ (as opposed to a single signal for each of ligands 1 – 3),
the signal corresponding to the PTA phosphorus entity, which shifts upfield, and a septet for
the PF6¯ phosphorus entity.
Synthesis and characterisation of the Rh(I)-PTA complexes (4 – 6)
The mononuclear (4), binuclear (5) and trinuclear (6) complexes were prepared in order to
investigate the effect of increasing the number of metal centres on the activity and
selectivity in the hydroformylation of 1-octene. Ligands 1’, 2’ and 3’ were each reacted with
[RhCl(COD)]2 to afford complexes 4, 5 and 6 respectively (Scheme 3). The complexes were
obtained as yellow solids in good yields (61 – 94 %). The metal complexes are sparingly
soluble in water at room temperature, largely due to the highly hydrophobic PF6¯
counterions. Solubility studies conducted at 40 °C in water gave an average solubility of
2 mg/mL for complexes 4 – 6.
1
Coordination of the ligands to the metal centre is confirmed by three new signals in the H
NMR spectrum of each of complexes 4, 5 and 6 accounting for the COD protons.
Coordination of the ligands via the phosphorus atom is further corroborated by the 31P{1H}
NMR spectra. Each of the 31P{1H} NMR spectra of complexes 4, 5 and 6 show doublets at
δ = -32.67 with coupling constant JRh-P = 136.0 Hz, at δ -32.17 with coupling constant
JRh-P = 157.9 Hz and δ = -32.38 with coupling constant JRh-P = 145.5 Hz respectively. The
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ESI-MS(+) spectrum for complex 4 shows a base peak at m/z = 264.1263 corresponding to
adduct [M–PF6+Na]2+. A base peak is observed at m/z = 312.9192 for complex 5
corresponding to [M–2PF6+Na]3+ and the mass spectrum for complex 6 shows a base peak at
m/z = 476.9400 corresponding to [M–3PF6]3+.
Scheme 3: Synthesis of the Rh(I)-PTA catalyst precursors (4 – 6) and their carbonyl analogues (7 – 8).
The carbonyl (CO) analogues 7 and 8 were synthesised following a published literature
method with minor modifications (Scheme 3) [35–37]. The rationale for the preparation of
these complexes is based on different electronic and steric properties that arise from the
COD and CO ligands. CO is a good π-acceptor and hence electron density can be back-
donated from the metal centre to the CO ligand, leaving the metal centre electron deficient.
Thus, the electron density around the rhodium of the complexes bearing the CO ligands is
expected to be less than the electron density around the rhodium of the complexes bearing
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the COD ligands. Moreover, CO is a less bulky ligand than the COD ligand and would possibly
influence the regioselectivity during the catalytic reactions in a different manner to the COD
ligand.
Each of complexes 4 and 5 were reacted with gaseous CO (Scheme 3) to afford complexes
7 and 8 as light orange solids in good to excellent yields (70 – 98 %). Despite several
attempts under varying conditions, including increasing the temperature and CO pressure,
the CO analogue of the trinuclear complex 6 could not be successfully isolated. This could be
due to the poor solubility of complex 6 in most organic solvents.
The successful synthesis of complex 7 was confirmed by the absence of the COD proton
1
signals and the slight upfield shift of the PTA proton signals in the H NMR spectrum. The
13C{1H} NMR spectrum of 7 further confirms the formation of this complex. A signal is
observed at δ = 192.08 assigned to the CO ligands. The signal corresponding to the PTA
phosphorus entity in 31P{1H} NMR also shows an upfield shift indicating successful
substitution of the COD ligand for the CO ligands.
Infrared (IR) spectroscopy further corroborates the substitution of the COD ligand with the
CO ligands. The presence of two ν(C≡O) absorpꢀon bands at 2048 and 1996 cm-1 suggest
that the two CO ligands are in different chemical environments. The two CO ligands are cis
to each other in solid state, i.e. one is trans to the chlorido ligand and the other is trans to
1
the PTA ligand. Similar shifts in the H, 13C{1H} and 31P{1H} NMR spectra and similar IR
patterns were also observed for complex 8. The ESI-MS(+) spectra show a base peaks at
m/z 248.1313 corresponding to [M–PF6+Na]2+ (for complex 7) and at m/z 268.1862
[M–2PF6+H]3+ for complex 8.
Aqueous biphasic hydroformylation of 1-octene
The mononuclear (4), 1,4-dinuclear (5) and the trinuclear (6) complexes were utilised for the
initial catalytic studies in order to determine optimal conditions for the aqueous biphasic
hydroformylation of 1-octene. These experiments were performed at temperatures of 50,
75 and 95 °C and 20, 30 and 40 bars of syngas pressure (CO:H2 = 1:1) in toluene (5 mL) and
water (5 mL). All catalytic reactions were carried out in duplicate for 8 hours. The catalyst
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loading was maintained at 2.87x10-4 mol% with respect to the rhodium metal centre
(Rh:substrate = 1:2500) for each catalyst precursor. Complexes 7 and 8 were then evaluated
as catalyst precursors in the hydroformylation of 1-octene at the optimal conditions
determined.
Table 1: Hydroformylation results of 1-octene over 8 hours.
c
Entry Catalyst
Pressure Temperature
Conversion Aldehydes
Iso-octenes
TOF
(h-1)
n:iso
a
b
b
d
(bar)
(°C)
(%)
(%)
(%)
Total
linear
branched
1
2
3
4
5
6
7
8
4
5
6
20
20
20
30
30
30
40
40
40
20
20
20
30
30
30
40
40
40
20
20
20
30
30
30
40
40
40
40
40
50
75
95
50
75
95
50
75
95
50
75
95
50
75
95
50
75
95
50
75
95
50
75
95
50
75
95
75
75
10
95
98
10
98
96
17
99
99
16
97
99
22
98
99
26
98
99
48
98
96
50
99
99
51
98
98
80
86
67
46
42
77
69
45
80
93
94
63
55
50
78
74
63
71
75
80
63
61
67
83
79
76
82
91
91
74
78
68
60
55
65
59
52
64
52
46
68
63
41
68
59
43
62
61
42
68
55
40
65
52
42
62
52
44
79
73
32
40
45
35
41
48
36
48
54
32
37
59
32
41
57
38
39
58
32
45
60
35
48
58
38
48
56
21
27
33
54
58
23
31
55
20
7
2.1
2.6
1.2
1.9
1.4
1.9
1.8
1.1
0.8
2.1
2.1
0.7
2.1
1.4
0.8
1.7
1.6
0.7
2.1
1.2
0.7
1.9
1.1
0.7
1.7
1.1
0.8
3.8
2.5
22
102
130
24
212
136
41
288
292
31
125
195
44
227
241
57
206
247
91
187
199
121
255
245
122
278
263
223
213
9
6
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
37
45
50
22
26
37
29
25
20
37
39
33
17
21
24
18
9
9
26
22
7
8
Reactions carried out in toluene (5 mL) and distilled water (5 mL) with 7.18 mmol of 1-octene and 2.87 x 10-3, 1.44 x 10-3 and 9.57 x 10-4
mmol of each of catalyst precursors 4, 5 and 6 respectively varying the temperature (50, 75 and 95 °C) and pressure (20, 30 and 40 bar)
of syngas (CO:H2 = 1:1). GC conversions were obtained using n-decane as an internal standard in relation to authentic standard iso-
a
b
octenes and aldehydes. TOF = (mol product/mol catalyst)h-1 and is based on total aldehydes. Average error estimates: ±2.1, ±1.5,
c
d
±0.1 % and ±4.2 %
Effect of temperature
The reactions were carried out at 50, 75 and 95 °C. At 50 °C, all three catalyst precursors
exhibit low conversion, with the highest conversion being 51 % (with complex 6) at 30 bar.
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In general, the conversion of 1-octene largely increases (>90 %) when the reactions are
carried out at 75 and 95 °C. For 5 and 6, a temperature of 75 °C favours linear aldehydes
whereas at 95 °C branched aldehydes are favoured. When catalyst precursor 4 is used,
linear aldehydes are favoured both at 75 and 95 °C. Thus, when using 5 and 6 as pre-
catalysts for the hydroformylation of 1-octene, the temperature should be selected
according to the type of desired aldehyde (linear or branched).
Effect of pressure
Reactions were carried out at 20, 30 and 40 bars. For catalyst precursors 4 and 6 the
reactions carried out at 40 bars exhibit higher chemoselectivity (up to 93 % aldehydes with
complex 4 at 75 °C) than at 20 and 30 bars. Catalyst precursor 5 exhibits a minimal
formation of the hydrogenation product, octane (8 % and 14 % at 75 and 95 °C respectively)
at a pressure of 40 bars.
Effect of increasing metal centres
There is no significant difference in hydroformylation results obtained (conversion and
selectivity) when using catalyst precursors 4 and 5. When the catalyst precursor 6 is
employed, a significantly higher conversion of 1-octene is observed at 50 °C (up to 51 % at
30 bar) when compared to the very poor conversion seen when using 4 and 5 (highest being
26 % with 5 at 40 bar). This implies that the catalyst 6 exhibits faster rates than the
mononuclear (4) and binuclear catalysts (5).
Optimal conditions
At low temperature (50 °C) the catalyst precursors exhibit poor conversion of 1-octene. The
optimal temperature was established to be 75 °C, which favours linear aldehydes over
branched aldehydes and the optimal pressure is 40 bar, which in turn exhibits better
chemoselectivity towards aldehydes. All three complexes exhibit good catalytic activity at
these optimal conditions, with TOF values ranging between 200 and
300 h-1.
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Performance of COD complexes (4 – 6) and CO complexes (7 and 8) compared
All complexes exhibit very good to excellent conversion (greater than 80 %). The COD
containing catalyst precursors (4 – 6) show greater conversion and are generally more active
(with TOF values as high as 288 h-1, entry 8) than 7 and 8, the dicarbonyl analogues. This
could be attributed to the difference in electronic properties of the COD and CO ligands.
Further studies would have to be conducted in order to determine the mechanism of the
catalytic reactions when using catalyst precursors 4 – 8. This would assist in further
explaining the difference in conversion of 1-octene when using the catalyst precursors
(4 – 6) and the CO pre-catalysts (7 and 8).
The chemoselectivity of the catalyst precursors at optimal conditions is displayed graphically
in Figure 2. The COD complexes (4 – 6) display better chemoselectivity for aldehydes
compared to the CO catalysts (7 and 8). This is attributed to the COD complexes reaching
near quantitative conversion after 8 hours, as a result, most of the iso-octenes that have
formed are hydroformylated leading to a decrease in the amount of iso-octenes present and
an increase in the amount of total aldehydes produced. No hydrogenation products (alkanes
and alcohols) are observed for any of the catalyst precursors.
Chemoselectivity
100
80
60
Iso-octenes
40
Total aldehydes
20
0
4
5
6
7
8
Catalyst precursors
Figure 2: Chemoselectivity of the catalyst precursors studied for the aqueous biphasic
hydroformylation of 1-octene.
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The CO catalyst precursors (7 and 8) display better regioselectivity compared to the COD
containing catalyst precursors (4 – 6) at optimal conditions (75 °C and 40 bar), as shown by
the graph in Figure 3. The n:iso ratios of the COD containing catalyst range between 1.1 and
1.6 whilst the n:iso ratios of the CO complexes go as high as 3.8 (entry 28). This is consistent
with results obtained by Makhubela et al. when comparing regioselectivity exhibited by
rhodium pre-catalysts that have COD and CO as ligands [38]. This means that the dicarbonyl
complexes are more selective for linear aldehydes than the COD complexes, a result that
was not expected as the COD ligand is bulkier than the carbonyl ligand and thus catalyst
precursors with COD ligands would be expected to exhibit selectivity towards linear
aldehydes than the complexes with the CO ligands. This could be due to the fact that with
CO ligands the PTA ligand can move freely, as opposed to when a bulky COD ligand is in the
vicinity. Such movement may result in desirable steric effects around the rhodium metal
centre, therefore favouring linear aldehydes (i.e. high n:iso ratios).
Regioselectivity
100
80
60
B-aldehydes
40
l-aldehydes
20
0
4
5
6
7
8
Catalyst precursors
Figure 3: Regioselectivity of the catalyst studied for the aqueous biphasic hydroformylation of 1-
octene.
Performance of catalyst precursors over time
No significant difference in the activity and selectivity is observed with an increase in the
number of metal centres. This motivated the study of the performance of catalyst
precursors 4 – 6 at different time intervals. The performance of each of catalyst precursor
4 – 6 was studied at different time intervals under the optimal catalytic conditions (75 °C
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and 40 bar). A sample was taken and analysed by GC after 1, 2, 4 and 8 hours for each
catalyst precursor. Figure 4 illustrates the conversion profile of the catalyst precursors with
time.
Conversion as a function of time
100
90
80
70
60
4
5
6
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
Time (h)
Figure 4: Conversion profile with time for catalyst precursors 4 – 6.
The results show that at each time interval the trinuclear catalyst precursor (6) exhibits
higher conversion than all the other, while the dinuclear catalyst precursor (5) exhibits
higher conversion than the mononuclear catalyst precursor (4). When using 6 as a catalyst
precursor, maximum conversion is achieved after 4 hours. This infers that the trinuclear
complex exhibits a faster rate than the dinuclear complex which, in turn, exhibits a faster
rate than the mononuclear complex during the catalytic reactions.
Recyclability and leaching studies
Recyclability studies were performed by cooling the reaction mixture to 0 °C and the
toluene layer decanted. A fresh sample of 1-octene and n-decane dissolved in toluene was
then added onto the aqueous layer and the hydroformylation reaction repeated. The results
of the recyclability studies are shown in Figure 5. The catalysts could be recycled three times
with the conversion of 1-octene decreasing drastically after each run for each catalyst
precursor. The conversion had decreased to below 25 % by the third run. This decrease in
conversion could be attributed to the low concentration of the catalyst precursors in the
aqueous layer as a result of the catalyst precursors leaching into the organic layer.
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Catalyst Recyclability
100
90
80
70
60
50
40
30
20
10
0
Run 1
Run 2
Run 3
4
5
6
7
8
Complexes
Figure 5: Recyclability studies of the catalysts at optimal conditions.
Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to quantify
the loss of the catalysts 4 – 8 into the organic layer by analysing the aqueous and organic
layers of each catalyst before and after the hydroformylation reaction. The results show that
more than 90 % of each catalyst leaches into the organic layer after the first run. These
results corroborate the notion of leaching which then led to the observed significant drop in
the conversion after the first run (Figure 5). The huge loss into the organic layer could be
due to the catalyst precursors being soluble in toluene at elevated temperatures and
pressures.
Mercury poisoning studies
Mercury poisoning studies were conducted as the need to suppress unwanted
heterogeneous catalysts (nanoparticles) and thus determine whether the catalyst
precursors are entirely homogeneous under the catalytic conditions is important. Free metal
particles can be responsible for heterogeneous catalysis and therefore not giving a true
reflection of the performance of the catalyst precursors as homogeneous catalysts.
Mercury(0) is added to the catalytic reactions and can then form an amalgam with any free
metal particles present, thus inhibiting the activity of the heterogeneous catalysts which
may also exist as nanoparticles.
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A drop of mercury was added to the reactor at the beginning of the hydroformylation
reaction (t = 0 hours) and the catalytic reaction conducted under optimal conditions (75 °C
and 40 bar) for 8 hours using each one of catalyst precursors 4 – 8. The layers were then
separated and the organic layer analysed using GC. Figure 6 shows the results obtained from
these experiments. The conversion of 1-octene in the presence of mercury agrees (within
experimental uncertainty) with the conversion of 1-octene obtained without mercury for all
complexes employed. This suggests that only the molecular species is responsible for the
conversions observed and not nanoparticles, thus all complexes (4 – 8) behave entirely as
homogeneous catalysts under the catalytic conditions.
Mercury poisoning
100
80
60
Without Hg
With Hg
40
20
0
4
5
6
7
8
Complex
Figure 6: Comparison of 1-octene conversion in the absence and presence of mercury.
Conclusions
A series of mono-, di- and trinuclear Rh(I)-PTA complexes were synthesised and evaluated as
catalyst precursors for the hydroformylation of 1-octene in an aqueous biphasic medium. All
complexes were active in the hydroformylation of 1-octene to aldehydes, with TOF values
over 200 h-1. The COD complexes (4 – 6) proved to be more active than the CO complexes (7
– 8). Moreover, the COD complexes exhibit better chemoselectivity for aldehydes whilst The
CO complexes exhibit better regioselectivity for linear aldehydes. This is due to the CO
complexes not reaching quantitative conversion at 8 hours whereas all COD complexes
reach quantitative conversion under the catalytic conditions. Increasing the number of
metal centres had no significant effect on the chemo- and regioselectivity at 8 hours, but
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studies of the catalyst precursors over time showed that increasing the number of metal
centres increased the rate of the catalytic reaction. This is demonstrated by the trinuclear
complex displaying a faster catalytic rate than the dinuclear complexes which, in turn,
display faster rates than the mononuclear complexes. Mercury poisoning experiments
confirmed that all the catalysts behave entirely as homogeneous catalysts when used for
the aqueous biphasic hydroformylation of 1-octene. The complexes exhibited poor
recyclability, attributed to significant leaching into the toluene layer. ICP-OES studies
showed that ca. 90 % of each complex leaches into the toluene layer after the first run. The
leaching could be as a result of the complexes being soluble in toluene under the catalytic
conditions.
Experimental
Materials and methods
All reactions were carried out in an atmosphere of nitrogen unless otherwise stated.
Solvents were dried and degassed prior to use. All chemicals were purchased from Sigma
Aldrich and used as received, unless otherwise stated. RhCl3∙3H2O was purchased from
Heraeus South Africa. [RhCl(COD)]2 dimer as well as ligands 1-3 were prepared according to
previously reported procedures [32–34,39].
13
1H NMR spectra were recorded on a Varian-300 MHz and C{1H} and 31P{1H} were recorded
on a Varian-400 MHz (13C: 101 MHz, 31P: 162 MHz). Melting Points were obtained using a
BÜCHI melting point apparatus B-540. Mass spectrometry was performed using a Waters
Synapt G2 electron spray ionisation mass spectrometer in the positive or negative-ion mode
at Stellenbosch University and elemental analyses was performed using a Thermos Scientific
FLASH 2000 CHNS-O Analyzer at the University of Johannesburg. FT-IR spectra were
recorded using Attenuated Total Reflectance Infrared spectroscopy (ATR-IR). Catalytic
products were analysed and quantified using a Varian CP-8400 GC instrument. ICP-OES
Varian 750-ES spectrophotometer was used to conduct inductively coupled plasma optical
emission spectroscopy experiments
15

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General procedure for the synthesis of ligands 1’ – 3’
Either one of ligands 1 (101 mg, 0.356 mmol), 2 (199 mg, 0.407 mmol) or 3 (200. mg, 0.288
mmol) was dissolved in ethanol (50 mL). Appropriate equivalents of ammonium
hexafluorophosphate (69.2 mg, 0.425 mmol for ligand 1’; 140. mg, 0.859 mmol for ligands 2’
and 150. mg, 0.920 mmol for ligand 3’) were added and the reaction mixture stirred at room
temperature for 72 hours. The ligands precipitated out of solution as white solids which
were isolated by suction filtration and dried under vacuum.
1
Ligand 1’: Yield: 231 mg, 83 %. M.p.: 145.4 – 146.6 °C. H NMR (DMSO–d6, δ ppm): 7.67 –
7.44 (m, 3J = 4.0 Hz, 5H, ArH), 5.04 – 4.82 (AB q, 2J = 11.0 Hz, 4H, Hb), 4.47 – 4.33 (AB q, 2J =
13.2 Hz, 2H, Hd), 4.22 (d, 2J = 5.4 Hz, 2H, He), 4.07 (s, 2H, Ha), 3.93 – 3.76 (m, 2J = 11.4 Hz, 4H,
Hc). 13C{1H} NMR (DMSO–d6, δ ppm): 133.38 (Cg), 130.71 (Ci), 129.52 (Ch), 126.24 (Cf), 79.31
(Cb), 69.87 (Cd), 65.42 (Ca), 52.17 (Ce), 45.92 (Cc). 31P{1H} NMR (DMSO–d6, δ ppm): -83.50 (s,
1
PTA), -144.18 (sept, J = 711.3 Hz, PF6). ESI-MS(+): m/z 248.1320 [M–PF6]+. Elemental
Analysis: C13H19F6N3P2: Calcd. C 39.70, H 4.87, N 10.69 %. Found C 39.34, H 4.47, N 10.66 %.
S20°C = 4 mg/mL in H2O.
Ligand 2’: Yield: 262 mg, 91 %. M.p.: 182.4 – 185.4 °C 1H NMR (DMSO–d6, δ ppm): δ 7.61 (s,
4H, ArH), 5.06 – 4.85 (AB q, 8H, Hb), 4.59 – 4.33 (AB q, 4H, Hd), 4.24 (d, J = 4.4 Hz, 4H, He),
13
4.14 (s, 4H, Ha), 3.91 – 3.80 ppm (m, 8H, Hc). C{1H} NMR (DMSO–d6, δ ppm): 133.93 (Cg),
128.17 (Cf), 79.44 (s, Cb), 69.85 (s, Cd), 64.63 (s, Ca), 52.35 (d, Ce), 45.88 (d, Cc). 31P{1H} NMR
1
(DMSO–d6, δ ppm): -83.63 (s, PTA), -144.17 (sept, J = 711.2 Hz, PF6). ESI-MS(+): m/z
209.1084 [M–2PF6]2+. Elemental Analysis: C20H32F12N6P4: Calcd. C 33.91, H 4.55, N 11.86 %.
Found C 34.02, H 5.16, N 11.42 %. S20°C = 5 mg/mL in H2O.
Ligand 3’: Yield: 243 mg, 82 %. M.p.: 322.9 – 326.1 °C. 1H NMR (DMSO–d6, δ ppm): 77.5 (s,
3H, ArH), 5.27 – 5.09 (AB q, 2J = 11.0 Hz, 12H, Hb), 4.64 (d, 2J = 4.8 Hz, 6H, He), 4.50 – 4.11 (m,
2J = 16.5 Hz, 6H, Hd), 4.06 (s, 6H, Ha), 4.04 – 3.84 (m, 12H, Hc). 13C{1H} NMR (DMSO–d6, δ
ppm): 139.16 (Cg), 128.23 (Cf), 78.85 (Cb), 69.71 (Cd), 64.47 (Ca), 51.78 (Ce), 45.58 (Cc). 31P{1H}
1
NMR (DMSO–d6, δ ppm): -82.63 (s, PTA), -144.18 (sept., J = 711.2 Hz, PF6). ESI-MS(+): m/z
366.6320 [M–2PF6]2+. Elemental Analysis: C27H45F18N9P6: Calcd. C 31.68, H 4.43, N 12.32 %.
Found C 32.09, H 5.18, N 11.92 %. S20°C = 4 mg/mL in H2O.
16

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Synthesis of the mononuclear Rh-PTA metal complex (4)
The synthetic method reported by Hapiot and Gonsalvi [40] was followed (with minor
modifications) in the synthesis of complex 6. [RhCl(COD)]2 (63.5 mg, 0.129 mmol) was
dissolved in DCM (50 mL). Ligand 1’ (100. mg, 0.254 mmol) was then added and the reaction
mixture stirred at room temperature for 1.5 hours, after which the solution was
concentrated to ca. 10 mL. Cold pentane was then added affording complex 6 as a yellow
precipitate which was then isolated by suction filtration and dried under vacuum. Yield: 159
1
mg, 98 %. M.p.: Decomposes without melting, onset at 164.9 °C. H NMR (DMSO–d6, δ
ppm): 7.57 - 7.50 (m, 4H, ArH), 5.07 – 4.87 (AB q, 2J = 10.5 Hz, 4H, Hb), 4.77 (d, 2J = 10.8 Hz,
1H, Hd), 4.51 – 4.47 (m, 5H, Hcod, Hd), 4.33 (d, 2H, He), 4.22 (s, 2H, Ha), 4.12 – 3.94 (m, 4H, Hc),
3
13
2.45 – 2.19 (m, 4H, Hcod), 2.02 – 2.00 (m, J = 8.1 Hz, 4H, Hcod). C{1H} NMR (DMSO–d6, δ
ppm): 133.39 (Cg), 130.83 (Ci), 129.85 (Ch), 128.88 (Cf), 126.12 (Ccod), 78.75 (Cb), 64.35 (Cd),
31
68.09 (Ca), 55.35 (Ce), 52.12 (Cc), 27.98 (Ccod). P{1H} NMR (DMSO–d6, δ ppm): -32.67 (d,
1
1JRh-P = 136.0 Hz, PTA), -144.18 (sept., J = 711.2 Hz, PF6). ESI-MS(+): m/z 264.1263
[M–PF6+Na]2+. Elemental Analysis: C21H31ClF6N3P2Rh: Calcd. C 39.42, H 4.88, N 6.57 %.
Found C 39.72, H 5.17, N 6.61 %. S40°C = 3 mg/mL in H2O.
Synthesis of the 1,4-binuclear Rh-PTA metal complex (5)
[RhCl(COD)]2 (70.4 mg, 0.143 mmol) was dissolved in DCM (40 mL) and ligand 2’ (100. mg,
0.141 mmol) was then added. The reaction mixture was stirred at room temperature with
ligand 2’ in suspension for 2 hours. Complex 7 was observed as a yellow solid precipitating
out of solution. The complex was then isolated by suction filtration, washed with copious
amounts of methanol to get rid of unreacted ligand 2’ and then dried under vacuum. Yield:
1
159 mg, 94%. M.p.: Decomposes without melting, onset at 286.2 °C. H NMR (DMSO–d6, δ
ppm): 7.67 (s, 4H, ArH), 5.09 – 5.05 (m, 2J = 12.1 Hz, 8H, Hb), 4.91 – 4.75 (AB q, 4H, Hd), 4.54
– 4.50 (m, 8H, Hcod), 4.36 (d, 4H, He), 4.30 (s, 4H, Ha), 4.13 – 4.08 (m, 8H, Hc), 3.96 – 3.91 (m,
13
4H, Hcod), 2.41 – 2.15 (m, 8H, Hcod), 2.05 – 2.01 (m, 4H, Hcod). C{1H} NMR (DMSO–d6, δ
ppm): 134.09 (Cf), 130.37 (Cg), 127.97 (Ccod), 79.14 (Cb), 68.14 (Cd), 63.49 (Ce), 54.70 (Ca),
31
1
51.87 (Cc), 27.90 (Ccod). P{1H} NMR (DMSO–d6, δ ppm): -32.17 (d, JRh-P = 157.9 Hz, PTA),
1
-144.18 (sept., J = 711.2 Hz, PF6). ESI-MS(+): m/z 312.9192 [M–2PF6+Na]3+. Elemental
17

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Analysis: C36H56Cl2F12N6P4Rh2: Calcd. C 35.99, H 4.70, N 6.99 %. Found C 35.57, H 4.73, N
6.54 %. S40°C = 1.5 mg/mL in H2O.
Synthesis of the trinuclear Rh-PTA metal complex (6)
[RhCl(COD)]2 (72.6 mg, 0.147 mmol) was dissolved in DCM (50 mL). Ligand 3’ (99.6 mg,
0.0973 mmol) was then added and the reaction mixture was stirred at room temperature
for 24 hours. Complex 6 was observed as a yellow solid which was then isolated by suction
filtration, washed with DCM and dried under vacuum. Yield: 105 mg, 61 %. M.p.:
1
Decomposes without melting, onset at 359.4 °C. H NMR (DMSO–d6, δ ppm): 7.72 (s, 3H,
2
ArH), 5.31 – 5.13 (AB q, J = 10.5 Hz, Hb), 4.77 – 4.54 (m, 12H, Hd), 4.48 – 4.44 (m, 18H,
He, cod), 4.30 (s, 6H, Ha), 4.20 – 4.07 (m, 2J = 14.5 Hz, 12H, Hc), 2.38 – 2.22 (m, 12H, Hcod), 2.01
3
13
– 1.91 (m, J = 7.6 Hz, 6H, Hcod). C{1H} NMR (DMSO–d6, δ ppm): 138.33 (Cg), 128.45 (Cf),
31
126.89 (Ccod), 79.81 (Cb), 68.01 (Cd), 63.45 (Ca), 58.81 (Ce), 51.69 (Cc), 27.98 (Ccod). P{1H}
1
NMR (DMSO–d6, δ ppm): -32.38 (d, JRh-P = 145.5 Hz, PTA), -144.18 (sept., PF6). ESI-MS(+):
m/z 476.9400 [M–3PF6]3+. Elemental Analysis: C51H81Cl3F18N9P6Rh3: Calcd. C 34.74, H 4.63, N
7.15 %. Found C 34.63, H 4.97, N 7.75 %. S40°C = 1 mg/mL in H2O.
General procedure for the synthesis of complexes 7 and 8
Complex 4 (51.2 mg, 0.0800 mmol) and Complex 5 (200. mg, 0.166 mmol) were each
dissolved in acetone (40 mL) and gaseous CO was bubbled through the solution at room
temperature for 1 hour. The yellow solution gradually changed to orange. The solvent was
then reduced and cold pentane added to afford Complexes 7 and 8 respectively as orange
solids. The complexes were then each isolated by suction filtration, washed with pentane
and dried under vacuum.
1
Complex 7: Yield: 32.2 mg, 70 %. M.p.: Decomposes without melting, onset at 147.4 °C. H
NMR (DMSO–d6, δ ppm): 7.57 – 7.49 (m, 5H, ArH), 5.15 – 5.01 (AB q, 4H, Hb), 4.79 – 4.61 (AB
q, 2H, Hd), 4.42 (d, 2H, He), 4.25 (s, 2H, Ha), 4.13 – 3.97 (m, 4H, Hc). 13C{1H} NMR (DMSO–d6,
δ ppm): 192.08 (CO), 132.98 (Cg), 130.91 (Ci), 129.43 (Ch), 129.07 (Cf), 79.77 (Cb), 69.41 (Cd),
65.83 (Ca), 59.42 (Ce), 52.36 (Cc). 31P{1H} NMR (DMSO–d6, δ ppm): -33.44 (d, 1JRh-P = 125.3 Hz,
1
PTA), -144.18 (sept., J = 711.3 Hz, PF6). FT-IR: 2048 (CO), 1996 cm-1 (CO). ESI-MS(+): m/z
18

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248.1313 [M–PF6+Na]2+. Elemental Analysis: C15H19ClF6N3O2P2Rh: Calcd. C 30.66, H 3.26, N
7.15 %. Found C 30.80, H 3.71, N 7.79 %. S40°C = 4 mg/mL in H2O.
1
Complex 8: Yield: 178 mg, 98 %. M.p.: Decomposes without melting, onset at 232.8 °C. H
NMR (DMSO–d6, δ ppm): 7.76 – 7.62 (m, 5H, ArH), 5.19 – 5.10 (m, 8H, Hb), 4.92 – 4.73 (m,
13
4H, Hd), 4.45 - 4.30 (m, 8H, Ha,e), 4.21 – 4.01 (m, 8H, Hc). C{1H} NMR (DMSO–d6, δ ppm):
193.77 (CO), 134.29 (Cg), 130.22 (Cf), 79.16 (Cb), 68.93 (Cd), 63.77 (Ca), 58.69 (Ce), 51.67 (Cc).
1
1
31P{1H} NMR (DMSO–d6, δ ppm): -20.93 (d, JRh-P = 148.5 Hz, PTA), -144.17 (sept., J = 711.3
Hz, PF6). FT-IR: 1998 (CO), 2095 cm-1 (CO). ESI-MS(+): m/z 268.1862 [M–2PF6+H]3+.
Elemental Analysis: C24H32Cl2F12N6O4P4Rh2: Calcd. C 27.65, H 3.57, N 7.44 %. Found C 27.81,
H 4.03, N 7.05 %. S40°C = 2 mg/mL in H2O.
General procedure for the Hydroformylation reactions
The hydroformylation reactions were carried out in a 90 mL stainless steel pipe reactor. The
reactor was charged with toluene (5 mL), distilled water (5 mL) the substrate, 1-octene
(805 mg, 7.18 mmol), the internal standard, n-decane (204 mg, 1.44 mmol) and either one
of the Rh-PTA catalyst precursors 4 – 8 (Rh metal centre:substrate = 1:2500). The air-tight
reactor was de-aerated by flushing twice with H2 gas, then pressurised and heated to the
desired syngas (CO:H2, 1:1) pressure and temperature respectively. After 8 hours, the
reactor was depressurised and the reaction transferred to a vial and cooled to 0 °C, the
organic layer decanted and analysed using gas chromatography.
Acknowledgements
Financial support from the University of Cape Town (UCT) and University of Johannesburg
(UJ), the Department of Science and Technology of South Africa, the NRF-DST Centre of
Excellence in Catalysis – c*change and the Harry Crossley foundation is gratefully
acknowledged.
19

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Highlights
‹ Mono-, di- and trinuclear Rh(I) complexes based on alkylated-PTA scaffolds
have been prepared.
‹ The complexes were characterized using several spectroscopic and analytical
techniques.
‹ The complexes were evaluated as catalyst precursors for the hydroformylation
of 1-octene.
‹ The trinuclear complex showed higher conversion rates.
‹ Poor recyclability was achieved.